1. Field of Invention
This invention is related to determining the probable seating plane for an array of connection elements. In particular, this invention is directed to systems and methods for determining the seating plane of a ball-grid array.
2. Description of Related Art
As defined by the Joint Electron Device Engineering Council in the xe2x80x9cBall-grid Array Packagexe2x80x9d JEDEC STANDARD, No. 95-1, Section 14, (the Standard) incorporated herein by reference in its entirety, a ball-grid array is a square or rectangular area of metallized balls, columns or other connection structures formed on an array side of an integrated circuit package. The main body of the package has a metallized circuit pattern applied to a dielectric structure. One or more semiconductor dies are attached to a die side of the dielectric structure, which may be either the top or bottom surface of the dielectric structure. On the array side of the dielectric structure is an array pattern of the metallized balls, columns or other connection structures that form the mechanical and electrical connection from the package to a mating feature such as a printed circuit board. The surface which contains the semiconductor die may be encapsulated by various techniques to protect the semiconductor die.
The Standard defines a ball-grid array seating plane as the plane simulated by a horizontal surface that is in contact with the apices of three or more non-collinear balls that support the package when it is placed on the top of this surface. The triangle formed by the three or more balls defining the seating plane must include the center of gravity of the package. If multiple possible seating planes meet these conditions, then the potential seating plane with the worst coplanarity is defined as the actual seating plane, since this will emphasize the potential for xe2x80x9cout of planexe2x80x9d connection elements to lead to a connection failure.
A seating plane is used as a very important reference when evaluating dimensions and tolerances of a ball-grid array, such as the thickness of the ball-grid array package body, coplanarity and warpage. However, there is no method for seating plane determination proposed in the standard.
As mentioned above, the seating plane is used as a reference when evaluating dimensions and tolerances. For instance, the Joint Electron Device Engineering Council has proposed one method for determining the ball-grid array coplanarity. In this method, coplanarity is the distance between the seating plane and the apex of the ball/column which is the furthest from the seating plane among all balls. However, there is not a method to determine the seating plane as proposed in the Standard.
As a substitute approach, a second method for determining coplanarity first establishes a least mean square plane. The least mean square plane is determined by calculating the least mean square of the distance between the spherical crowns of all the balls or columns of the ball-grid array. A shifted least mean square plane is then determined by shifting the least mean square plane along the direction normal to the least mean square plane and away from the ball-grid array package until the apex of the ball or column having the greatest distance from the original least mean square plane lies on the shifted least mean square plane. This plane is called lowest ball reference plane and used as a substitute for the defined seating plane in the case of a coplanarity calculation.
Actually this substitute xe2x80x9cseating planexe2x80x9d is also used in cases of other dimension and tolerance calculations because there is no current method for determining the seating plane. Since the ball-grid array is becoming a major device in the semiconductor industry, there is a strong need for standards to guide design, manufacture and inspection and a need to comply with these standards.
The foregoing substitute methods do not consider the location of the center of gravity of the ball grid array. Furthermore, the least mean square plane by it""s nature tends to pick a xe2x80x9cleast tiltedxe2x80x9d plane (the average plane) based on the entire array of balls, whereas the seating plane, by it""s definition, is intended to contact only the apices of the balls. The tilt of a seating plane based on only on the most extreme apices has a high probability of being more tilted than the previously described least mean square plane. Furthermore, any measure of coplanarity, one of the most important parameters for determining ball grid array quality, depends very strongly on the tilt of the assumed seating plane. Thus, there is a strong need for a rigorous method of determining the seating plane as defined in the Standard.
This invention provides systems and methods that determine the ball-grid array seating plane in conformance with the Standard.
Accordingly, the systems and methods for determining the ball-grid array seating plane according to this invention enable a seating plane to be determined that is in accordance with the Joint Electron Device Engineering Council definition of xe2x80x9cseating plane.xe2x80x9d
Specifically, a ball-grid array package is presented to a ball-grid array ball measuring system. The ball-grid array ball measuring system measures the distance from a reference plane to the apex of each of the balls. These distances are then forwarded to the ball-grid array seating plane determining system. The nominal center of gravity, which could be based on design data, may also be forwarded to the ball-grid array seating plane determining system. The ball-grid array seating plane determining system determines the ball-grid array seating plane in accordance with the Standard.
Although the following description of the exemplary embodiments of the systems and methods according to this invention refer to a ball grid array, it should be appreciated that this is an exemplary application of the invention. In general, the methods and systems of this invention can be used for determining stable seating planes for any device resting on the most extreme points included within an array of mechanical or electrical connection elements. The apices of a ball grid array are referred to in the following description. However, in general, the apices of any type of array of connection elements should be considered equivalent to the apices of a ball grid array.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.